This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Proton T1[unreadable] MRI studies have shown that spin-locking can provide unique and clinically-useful contrast correlated to macromolecular content due to its ability to probe slow molecular motion. Although spin-locking spectroscopy of sodium has been investigated as a means to selectively detect sodium nuclei involved in slow molecular motion and anisotropic environments, to knowledge there is currently no literature reporting spin-lock sodium MRI data. A large fraction of sodium ions in biological tissue are complexed to macromolecules through electrostatic binding sites and their interactions with the bound water fraction. Several tissue pathologies, e.g. stroke and osteoarthritis, involve substantial changes to motional characteristics of the complexed fraction of sodium spins. Measurements of T2 or T2* are sensitive to slow motion only through the spectral density at zero frequency;slow dynamics are probed more efficiently through the application of a strong RF fields in the sub-kilohertz range. We hypothesize that spin-lock sodium MRI will generate significant contrast based on macromolecular content in biological tissue due to its sensitivity to slow molecular motion.